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Vibrational and Quantum Yield Studies of Deuterated Tryptophan Radical in Azurin


Long-range electron transfer has been observed in several complex biological systems, including photosynthetic complexes to redox enzymes in DNA repair pathways. The efficiency of electron tunneling and hopping through amino acid mediators, including tyrosine and tryptophan, has been reported in the past. In particular, tryptophan residues are redox active and can function as electron transfer (ET) intermediates through progressive conversion to cation and neutral radical. The current work extends upon the published studies of the tryptophan radical at position 48 of the zinc-substituted protein, azurin (denoted ZnIIAz48W) by focusing on the effect of isotopic substitution on electron and protein transfer steps. Two ZnIIAz48W isotopologues of trp-48 are studied here: perdeuterated D5- ZnIIAz48W and singly- deuterated ND- ZnIIAz48W. These isotopologues were characterized by UV- and visible-resonance Raman spectroscopy for both the closed shell and neutral radical forms, respectively. Isotope effects were also observed in the fluorescence and radical quantum yields. The fluorescence and phosphorescence intensities, in the presence and absence of an external electron acceptor, indicate that the triplet state may be involved in the ET and cation radical formation pathways. The quantum yield for formation of D5- ZnIIAz48W is consistent with ET from the triplet state. In the case of ND- ZnIIAz48W, ET also likely occurs from the triplet state, but there is an additional effect because deprotonation of the heavy deuteron reduces the quantum yield for radical formation by a factor of 0.67 relative to NH- ZnIIAz48W. These isotope studies of Trp-48 in azurin help clarify the sequential ET/PT steps for trp-48 in azurin, and provide general insights into this complex biological process.

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